Telomeres are little understood structures at the ends of eucaryotic chromosomes. Our specific aims are to determine the structure of telomeres, and the mechanism of telomere replication and resolution. We will carry out a deletion analysis of cloned yeast and Tetrahymena telomeres, and sequence the critical regions. We will attempt to construct new telomeres from synthetic oligonucleotides, and we will begin an in vitro analysis of telomere replication and resolution. We plan to construct artificial chromosomes using cloned telomeres, centromeres, origins of replication, and genes. This will allow us to determine all of the structural requirements for normal chromosome function in yeast, and to study chromosome behaviour in yeastwith small, easily manipulated chromosomes. We will also begin to examine the structure and function of telomeres from higher organisms. These studies will make use of recent advances in the molecular genetics of the yeast, Saccharomyces cerevisiae. This organism is an ideal eucaryote with which to work, since its genome can be manipulated at will by the experimenter. The information gained and the techniques developed in the course of this work should have many applications to the study of chromosome behaviour in higher organisms.